1. Field of the Invention
The present invention relates to a liquid-crystal shutter.
2. Description of the Related Art
A liquid-crystal device using plastic films or plastic plates as the substrates thereof is thin and lightweight in comparison to a liquid-crystal device using glass substrates. Also, it is possible for such a liquid-crystal device to have a curved outline and a curved surface. Therefore, such a type of liquid-crystal device has drawn attention because this type of liquid-crystal device is suitable as a liquid-crystal shutter, such as sunglasses or goggles, the transmittance of which is electrically adjustable, used in front of a human face, for example.
Because such a liquid-crystal shutter is not required to be driven in a high-rate time-sharing manner, a TN-type liquid crystal (90.degree.-twisted twisted-nematic type liquid crystal) is used as the liquid-crystal shutter. It is easy to obtain a high contrast from the TN-type liquid crystal.
Although it is preferable that plastic-film substrates of such a type of liquid-crystal device be optically isotropic, a completely optically isotropic plastic-film substrate has not been put to a practical use. Further, although it is possible to obtain a plastic-film substrate which is initially approximately isotropic, it is easy for the plastic-film substrate to have anisotropy developing therein due to deformation thereof caused by a stress being applied thereto or deformation thereof caused by temperature or humidity. Accordingly, it can be said that any plastic-film substrate is somewhat anisotropic in general.
When such a film substrate having optical anisotropy is used as the substrates of a liquid-crystal device, a bad influence of the anisotropy thereof can be reduced as a result of the retardation values of the plastic substrates being caused to be not more than 15 nm as disclosed in Japanese Laid-Open Patent Application No. 60-78420, or as a result of the directions of optical axis of the film substrates being caused to be coincident to or perpendicular to the directions of absorption axis of polarizing plates as disclosed in Japanese Laid-Open Patent Application No. 61-100726. The retardation value is a value of d.DELTA.n where .DELTA.n denotes the difference in refractive index between a direction perpendicular to and a direction parallel to the optical-anisotropy axis of the substrate, and `d` denotes the thickness of the substrate.
However, the retardation of a uniaxially-stretched film substrate is on the order of 1 .mu.m, and, is approximately the same as or larger than the retardation of a liquid-crystal layer. Therefore, it is necessary to cause the polarization axis of the polarizing plate and the lagging axis of the substrate to strictly coincide with one another. When the directions of the axes differ from one another even slightly, deterioration in contrast and/or brightness of the liquid-crystal device develops.
Further, in general, a plastic film is a so-called double-axis optically anisotropic body in which not only the refractive indexes in directions in the plane thereof differ from one another but also the refractive index in the thickness directions differs from the refractive index of a direction in the plane thereof. Therefore, the retardation value obtained when the film substrate is viewed from an oblique direction is different from that obtained when it is viewed from the front direction due to an influence of the refractive index nz in the thickness directions. Such a change in retardation of the film substrate due to a change in visual-angle direction causes a change in brightness due to a change in visual-angle direction and deterioration in contrast.
Due to recent improvement in manufacturing technology of plastic-film substrates, optical anisotropy in directions parallel to the plane of a substrate is reduced, and, it has been possible to manufacture a plastic-film substrate in which the optical-axis directions are within .+-.10.degree., and, also, the magnitude of retardation is not more than 30 nm, and, in some case, not more than 10 nm.
However, it is very difficult to control the refractive index in the thickness directions to a predetermined value, and, a substrate in which the refractive index in the thickness directions is maintained at a predetermined value has not been put into practical use.
Substrates for liquid-crystal devices are desired to have a durability against chemicals and solvents which are used in manufacturing processes thereof, a sufficient gas-barrier property and a sufficient surface-smoothness property, and so forth.
Accordingly, an arrangement of a plastic-film substrate is required such that, even when the refractive index nz in the thickness directions is smaller than the refractive indexes nx and ny in the directions in the plane thereof, and, thereby, the retardation value of the substrate itself changes due to a change in visual-angle direction, a liquid-crystal device made using such substrates has good visual-angle characteristics.
FIG. 1 shows a plan view of a state in which a human being 1 views objects 3 through a liquid-crystal shutter 2. A liquid-crystal shutter used in a condition of being arranged in front of the face of a human being is required to have the same brightness and contrast characteristics for both the eyes thereof. However, in general, a liquid-crystal shutter has a so-called visual-angle property such that brightness and contrast differ depending on direction in which light is transmitted thereby. Further, when a liquid-crystal shutter is used in a condition of being arranged in front of the face of a human being, the distance between the eyes and the liquid-crystal shutter is small. Thereby, the influence of the visual-angle property is large. Accordingly, the brightness sensed by one eye may be different from the brightness sensed by the other eye through the liquid-crystal shutter depending on the angle at which the human being 1 views the objects 3. When the brightness sensed by one eye is different from the brightness sensed by the other eye, this matter causes fatigue of the eyes.
Accordingly, it is important to cause the contrast characteristics of a liquid-crystal shutter used in front of the face of a human being for one eye of the human being to be the same as those thereof for the other eye of the human being. When a liquid-crystal shutter is used in front of the face of a human being, a portion thereof corresponding to the nose of the human being is constricted so that the liquid-crystal shutter has a shape of glasses, as shown in FIG. 2 which shows a front elevational view of the liquid-crystal shutter. As shown in FIG. 2, the liquid-crystal shutter includes right and left display portions (transparent-electrode opposing portions) 13a and 13b, an extended electrode portion 14, and a constricted portion 15. FIG. 2 also shows lagging-axis directions 106 of upper and lower substrates, lagging-axis directions 107 of retardation developing due to a stress as described later, and long-axis directions 108 of the liquid-crystal shutter (which directions are directions parallel to the longest side of the periphery of the liquid-crystal shutter) and short-axis directions 109 thereof (which directions are directions perpendicular to the long-axis directions).
When a liquid-crystal shutter has the above-described arrangement, the stress due to shrinkage of polarizing plates thereof and so forth concentrates at the constricted portion 15, and, thereby, retardation develops in substrates thereof (the retardation develops in the directions 107 in FIG. 2).
Shrinkage of polarizing plates is larger in the long-axis directions than that in the short-axis directions. Therefore, retardation developing in substrates in the middle of a liquid-crystal shutter develops in the directions 107 and causes a decline in contrast. By this reason, it is not possible to easily achieve a liquid-crystal shutter having a constricted shape as shown in FIG. 2. Similarly, achievement of a liquid-crystal shutter having a curved-surface shape has a problem in that development of retardation in substrates thereof due to stress results in decline in contrast.